Electroadhesion grippers are used to retain workpieces on an adhesion face (gripping face) of the gripper, for instance, for transporting the workpieces in industrial production processes, in particular linked production processes.
There are constructed on the adhesion face two electrodes, to which a voltage is applied so that there is a powerful electrical field between the electrodes. A workpiece which abuts the adhesion face remains bonded to the adhesion face when the electrical field is switched on.
GB 1352715 A (Stevko) discloses an electroadhesion gripper in which the two electrodes are constructed with mutually engaging, straight fingers. In an alternative embodiment, the electrodes are constructed as two spirals which are fitted one in the other.
Electroadhesion is suitable in particular for retaining workpieces which are light in relation to the surfaces thereof, for example, metal sheets, because the active forces (compared, for example, with suction grippers or magnetic grippers) are relatively small.
WO 2008/070201 A2 (SRI International) discloses electroadhesion devices having a deformable adhesion face, in particular as auxiliary climbing means for scaling a rough wall. In addition to rake-like electrodes, electrodes constructed from concentric rings are also mentioned. Furthermore, there is proposed a hybrid gripper having a bell-like suction member, on the periphery of which an electroadhesion device is arranged in order to reduce a leakage rate and to increase the suction forces. WO 01/32114 A1 discloses a skin gripping apparatus, in which the skin of a human acting as an electrode is connected with respect to one or more electrodes of the apparatus. In a variant, a reduced pressure is further applied to a large number of air channels as a vacuum massaging device.
In particular when electroadhesion is used alone to grip and move a workpiece and only a correspondingly small force acts on the workpiece, an examination should be carried out in linked production processes as to whether the gripping of the workpiece was successful and whether the workpiece has been lost (dropped) during the movement action. It should further be examined whether the deposit of a workpiece was successful because capacitive residual charges at the electrodes or the workpiece can also bring about a substantial residual adhesion effect after the voltage has been switched off.
In order to ensure the discharge of a workpiece, it is known to support the discharge process in the case of an electroadhesion gripper with a discharge actuator, cf. GB 2354111 A. It is further known to determine capacitive residual charges with complex circuits and to discharge them, cf. JP 2007-019524 A, JP 6244270 A, JP 10284583 A, JP 11040661 A, so that the discharge can be carried out reliably; however, this procedure is complex and difficult in practice.
DE 10 2010 040 686 B3 discloses the monitoring in a suction gripper of the occupation with a workpiece by electrical measurements. In particular, an electrical contact can be closed or a parallel current path can be opened by a gripped metal workpiece; it is also proposed to evaluate an increase in the inductivity brought about by a gripped workpiece in a conductor loop near the workpiece position.
In order to check the occupation of an electroadhesion gripper, an additional measurement current circuit could be set up, which would, however, complicate the structure of the electroadhesion gripper and make the electroadhesion gripper substantially more expensive.